Synthesis And Mechanical Properties Of Nanoporous Aluminum And Its Alloys

Due to the "size effect",compared with their fully dense materials,bulk porous materials assembled by submicron or nano-scale objects into spatial network structure show superior mechanical and functional properties.Based on the superior properties and applications of nanoporous metals,various nanoporous metals have been prepared by dealloying,which is a simple and convenient method.However,most of previous nanoporous metals are limited to noble metals,and their high cost and high density seriously limit the relevant research and applications.Synthesis of non-noble nanoporous metals by dealloying is challenging,especially samples with high-quality(crack-free),because they are too reactive.But the research and applications of non-noble nanoporous metals are always in progress,hoping to fabricate nanoporous metals with superior properties,economic applicability and wide applications.It has been long expected that synthesis and performance studies on nanoporous Al will be undertaken,not only for the low cost,light weight but also for all the benefits arising from a stable and dense oxide layer that naturally occurs on the Al surface.Synthesis of nanoporous Al by dealloying in aqueous solution is challenging,because Al itself is so reactive that the nano-scale A1 ligament may be fully oxidized under this circumstance.In addition,synthesis of nanoporous Al by dealloying in aqueous solution also requires dissolution of even more-reactive elements,which may lead to a surface coverage of oxides and even hydrogen evolution process,which impede the dealloying process or deteriorate the obtained samples.In this paper,we propose a new procedure for synthesis of bulk nanoporous Al and Al-based alloys with native oxide layer,it involves both dealloying and galvanic replacement reaction(GRR),and is performed in a Lewis acidic AlCl3-[EMIM]Cl ionic liquid electrolyte.And their microstructure,formation mechanism and properties are characterized.This method can also be used for fabricating bulk non-noble nanoporous metals and alloys,and nanoporous metals with low density and high strength.The main results are summarized as below:1.Synthesis and microstructure characterization of nanoporous Al1)Nanoporous Al was prepared by dealloying/GRR of Al2Mg3 alloy in ionic liquid.The dealloying process generates a network nanoporous Al skeleton,and simultaneously the inward-growth plating of the reduced Al that thickens the Al skeleton.The obtained nanoporous Al has a uniform,isotropic and crack-free structure with an average ligament size of?200 nm and a relative density of?69.7%,and its macroscopic volume change before and after reaction is negligible.The ligament is nanocrystalline and the grain size is equivalent to the ligament size in nanoporous Al.There is an amorphous oxide layer on the surface of ligament with an average thickness of?10 nm,the ligament is in fact an Al-Al2O3 core-shell structure.2)Nanoporous Al was prepared by GRR of pure Mg,and the inward-growth ofdeposited Al happened without dealloying process.The obtained nanoporous Al has a fiber-like structure with lower relative density and poor connectivity,which is perpendicular to sample surface,and there is an oxide layer on the surface of ligament.The difference between the nanoporous Al samples prepared by pure Mg and Al2Mg3 shows that dealloying is critical for the formation of isotropic nanoporous structure.3)During GRR process,the inward-growth of the reduced Al may be caused by the formation of an electrically insulating amorphous alumina layer on the surface of ligament and outer surface of sample in ionic liquid.Al may not be deposited on the alumina layer.Most likely,the reduced Al deposited on ligaments at the reaction front before the alumina layer was formed.2.Thermal stability and mechanical properties of nanoporous Al synthesized by Al2Mg31)Due to the native oxide layer on the surface of ligament,nanoporous Al has excellent thermal stability.When nanoporous Al was annealed at 600? for 30 min in Ar,its ligament and grain size can remain stable.But higher annealing temperature or longer annealing time can cause coarsening of the structure.After annealing at 700? for 30 min,despite slight sample curling,nanoporous Al remained intact and some Al"droplets" were repelled out of the sample surface,and bulging occurred often but not always on samples.When nanoporous Al was annealed at 650? and below in Ar,the oxide layer can maintain amorphous,but densification occurred.2)Nanoporous Al is plastic under compression,and its yield strength is ? 73 MPa.When nanoporous Al was annealed at 600? and below in Ar,its yield strength increases with increasing annealing temperature.Nanoporous Al is fracture under tension,the fracture strength also increases with increasing annealing temperature.The ultimate tensile strength of annealed nanoporous Al is comparable to that of fully dense pure Al cold-rolled by 88%,and 2-5 times stronger than the annealed coarse-grain pure Al.The Young's modulus of nanoporous Al increases monotonically with increasing annealing temperature.Due to the densification of oxide layer,the strength of nanoporous Al increases after annealing at 600? and below.After annealing at 650?,the strength of nanoporous Al decreases slightly due to the softening of Al ligament caused by structural coarsening.The density of nanoporous Al is lower than that of most previous reported nanoporous metals.In contrast,nanoporous Al is among the strongest nanoporous metals ever reported in previous studies,both under tension and compression.And its specific strength is higher than that of conventional foam metals,foam Al-Al2O3 composites and nanoporous metals.The structure of nanoporous Al is less efficient in load-bearing than that proposed by Gibson-Ashby model.This presents the possibility for further stiffening and hardening of nanoporous Al samples in the future by optimizing their geometrical structures and further refining the structural sizes.3)When nanoporous Al was compressed to a strain of 0.30,most of the pore channels remain open,and localized slip bands are rarely observed in the deformed Al ligaments.Further examinations reveal dislocations trapped in some grains after compression.The dislocation density is relatively low and very little dislocation tangle is observed in the nanoporous Al after compression.Compared with grain boundaries,the alumina layer might have contributed more significantly to the strength of nanoporous Al.4)When nanoporous Al was annealed in air,the thickness of the oxide layer on the surface of ligament hardly changes at 400?,and increases with increasing annealing time at 600?.The thickening rate of the oxide layer slows down with annealing time at 600?.After annealing at 600? in air,the oxide layer of nanoporous Al is crystallized due to the exothermic oxidation of Al,which may cause local temperature exceeds the crystallization temperature of amorphous alumina.3.Synthesis and characterization of Al-based nanoporous alloys1)Nanoporous Al-Zn was prepared by Mg95Zn5 in ionic liquid at room temperature.The obtained nanoporous Al-Zn has an isotropic structure with an average ligament size of?44 nm and a relative density of?0.49,and there is an oxide layer on the surface of ligament.Nanoporous Al-Zn contains a lot of cracks,and shows excellent thermal stability.Furthermore,minor alloying of Zn can effectively refine the structure size,and prepare nanoporous materials with lower relative density.2)Nanoporous Ni-Al was prepared by Mg75Ni25 in ionic liquid at 50?.The obtained nanoporous Ni-Al has a uniform,isotropic structure with an average ligament size of?20 nm,and contains high density micro-cracks.There is an oxide layer on the surface of ligament,and Ni phase may exist in ligament.3)Nanoporous Ag-Al was prepared by Mg76Ag24 in ionic liquid at 250?.The obtained nanoporous Ag-Al has a uniform,isotropic structure with an average ligament size of?53 nm and a relative density of?0.41.Its grain size is equivalent to the ligament size.Nanoporous Ag-Al is not stable in air due to oxidization.During the dealloying/GRR process,the dealloying process generates a network nanoporous Ag skeleton,and simultaneously the inward-growth plating of the reduced Al that diffuses into Ag skeleton and forms Ag2Al.The ligament structure of nanoporous Ag-Al is Ag2Al-Al core-shell structure,and there is an oxide layer on the surface of ligament.Nanoporous Ag2Al can be prepared by immersing the as-prepared nanoporous Ag-Al in NaOH.The obtained nanoporous Ag2Al is intact with a uniform and well-connected structure,and its volume change is negligible.